Last month, Belgium advised its citizens to leave Burundi, while the EU cut staff levels, temporarily evacuating employees' "families and part of the non-essential staff".

"As a result of continuing violence, the Department of State ordered the departure of dependents of US government personnel and non-emergency US government personnel from Burundi," the US statement read.

"The US Embassy is able to offer only very limited emergency services to US citizens in Burundi."

It also gave advice for citizens who may encounter violence, advising them to stay indoors in ground floor rooms, away from doors and windows.

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Media captionThe BBC's Alistair Leithead: "The dead have been appearing nearly every day"

Col Gaspard Baratuza said 87 people were killed in clashes in the country's capital, Bujumbura, on Friday - 79 "enemies" and eight soldiers and policemen.

Witnesses told AFP that some of those killed were shot execution-style with their arms tied round their backs.

Carina Tertsakian, HRW's researcher for Burundi, called for a "serious and independent" enquiry into the killings.

"This is by far the most serious incident, with the highest number of victims, since the start of the crisis in April," she said.

"A serious, independent investigation is urgently needed to find out the exact circumstances in which these people were killed."

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Witnesses said men were shot "through the top of the skull"

BBC Africa analyst Richard Hamilton said bodies on the streets were almost a daily occurrence in Bujumbura, but that this was the largest number of deaths in one night.